In the past, much effort has been expended in preparing thermally stable polycarbonate compositions which would be color stable at elevated temperatures and particularly at the high molding temperatures generally employed to prepare molded polycarbonate articles. Many different additives have been found that are quite suitable for rendering polycarbonates heat and color stable. Particularly useful are triorgano-phosphites which have been disclosed in U.S. Pat. No. 3,305,520. Also, U.S. Pat. No. 3,729,440 discloses a thermally stable aromatic polycarbonate containing a phosphinite and an epoxy compound. Further, U.S. Pat. No. 3,673,146 discloses a method for stabilizing polycarbonate resin involving using a triorganophosphite, cycloaliphatic epoxy compound, ultramarine pigment and aryl siloxane fluid.
Polycarbonate polymers are excellent molding materials as products made therefrom have high impact strength, toughness, high transparency, wide temperature limits (high impact resistance below -60.degree. C. and a UL thermal endurance rating of 115.degree. C. with impact), good dimensional stability, high creep resistance and electrical properties which qualify it as sole support for current carrying parts.
Polycarbonates are, however, very difficult to fabricate from melts for the reason that melts thereof have exceptionally high viscosities. Attempts to overcome this difficulty by the incorporation with the polycarbonate of materials known to reduce the viscosity of other resins have very generally been unsuccessful. Many standard viscosity control agents appear to have little or no effect on the vicosity of polycarbonate. Other compounds known to lower the viscosity of resins cause degradation of polycarbonate resins. Some compounds, conventionally employed to improve the workability of polymers, produce an embrittling effect on polycarbonates when they are mixed therewith and the resin is subjected to elevated temperatures as in molding. Still other materials, while satisfactory stiffness modifying agents for other plastics, are too volatile to be incorporated with polycarbonates since polycarbonates have much higher melting points than many other thermoplastics.
Combinations of aromatic carbonate polymers and polysiloxanes have been disclosed in the prior art. Goldberg (U.S. Pat. No. 2,999,845), for example, discloses polycarbonate polysiloxane copolymers wherein the polysiloxanes employed in the copolymer are useful in obtaining a moiety that is not merely mixed with the polycarbonate, but is part of the polycarbonate molecule as a copolymer.
Caird (U.S. Pat. No. 3,087,908) discloses a polycarbonate polysiloxane composition to facilitate formation of clear films. Further, Bostick (U.S. Pat. No. 3,751,519) describes a composition containing a polycarbonate and a cyclosiloxane to improve mold release properties.
In general, these prior art silicon-containing compositions include a polysiloxane moiety, i.e., the --Si--O--Si-- linkage.